Conventionally, according to electronic blood pressure meters based on the oscillation method, diastolic and systolic blood pressure values are measured as described in the following. First of all, a cuff is applied to the upper arm of the subject and the air in the cuff is pressurized by a pump or the like to apply pressure to his upper arm and temporarily block the blood flow in the artery therein.
As the air in the cuff is gradually released at a constant rate, a variable component is found in the air pressure in the cuff (which is referred to as "cuff pressure" hereinafter) as shown in FIG. 8(c). This variable component which is called as pulse wave represents the change in the internal volume of the artery, and is transmitted from the artery to the cuff through the soft tissues of the upper arm as a variation in pressure.
A series of amplitude values Ap are separated and computed from the pulse wave through a filtering process (refer to FIGS. 8(b) and 8(c)). A maximum amplitude value Ap.sub.max is extracted from the amplitude values Ap, and an amplitude value Ap.sub.0.7 corresponding to a 70% level of the maximum amplitude value Ap.sub.max occurring before the occurrence of Ap.sub.max is also extracted. The cuff pressure Pc at the occurrence of Ap.sub.0.7 is determined as a systolic blood pressure value SYS.
Meanwhile, a critical point A.sub.PD is detected as an amplitude value corresponding to the point of change from a rapid decrease to a gradual decrease of the pulse wave amplitude Ap after the occurrence of the maximum pulse wave amplitude value Ap.sub.max. The cuff pressure Pc corresponding to the occurrence of A.sub.PD at t.sub.D is determined as a diastolic blood pressure value DIA.
Since the above described process of blood pressure determination is based on a statistical principle, it is not entirely free from measurement errors when the subject is an aged person or a person suffering from high blood pressure. Further, the critical point A.sub.PD is not very clear depending on the subject, and it may often happen that the determination of a diastolic blood pressure value DIA is not possible.